This invention relates to a process for reducing the benzene content of hydrocarbon fractions.
It is important to reduce the benzene content of petroleum products, in particular those of gasolines. Actually, benzene, although having a high octane number, is a toxic and volatile compound. Thus, the U.S. Environmental Protection Agency "EPA" has published a simple reformulated gasoline model that refiners have had to use since 1995 and that sets, among other things, the maximum amount of benzene in gasolines at 1% by liquid volume.
The benzene that is present in the gasolines comes from native benzene that is present in crude petroleum and benzene that is synthesized during various refining operations. In particular, the ratio of benzene that is provided by the catalytic reforming effluents is estimated at about 75% of the total benzene that is present in the gasolines.
To reduce the benzene content of gasolines, refiners adopt several strategies; they can:
pre-fractionate the catalytic reforming batch to eliminate a large portion of the benzene and its precursors (paraffins and naphthenes) at the top of the pre-fractionating column and to obtain, at the bottom, a C.sub.7+ fraction that is low in benzene and that feeds the reforming; in this case, however, the hydrogen yield of the reforming will also be reduced; PA1 modify the operating conditions of reforming by reducing the harshness of the treatment (this reduces the octane number of the gasolines, however); or else PA1 post-fractionate the reformate to obtain at the top of the post-fractionating column a light benzene-rich fraction and, at the bottom, a C.sub.7+ fraction that feeds the fuel pool. PA1 a) A distillation column is fed at least by a mixture of hydrocarbons that contains benzene and other hydrocarbons, for example, 5 to 10 carbon atoms, with said column making it possible to separate a distillate and a residue; PA1 b) at at least one point, a flow that is richer in benzene than the batch is drawn off on said distillation column; and PA1 c) said flow is sent to at least one permeation device that comprises a membrane that is selective for benzene, in which are separated a retentate that is low in benzene, at least part of which is sent to said column at at least one point on the latter, and a benzene-enriched permeate.
The benzene that is recovered at the top of the pre-fractionation or post-fractionation can be either extracted with a solvent and used as a petrochemical base, or converted by alkylation or by hydrogenation and/or hydroisomerization, then sent to the fuel pool.
These different treatments are disadvantaged, however, because of the low benzene content of the effluents that are obtained at the top of the pre-fractionating column or post-fractionating column.
One way of obtaining an effluent that is more concentrated in benzene is to carry out lateral drawing-off on the pre-fractionating column or post-fractionating column, advantageously at the level where benzene concentration exhibits a peak. The benzene contents of this drawing-off not exceeding 10 to 30% by weight, however, a relatively high drawing-off flow is necessary to be able to decrease the benzene at the top and the bottom of the column in a significant way.